UB 305×102×33 vs H-300x150x5.5x8
UB 305×102×33 is an UB section from the UK Blue Book (SCI P363); H-300x150x5.5x8 is the closest equivalent in the European (Continental) range. They are close to interchangeable in bending: 481 cm³ against 475 cm³ of plastic modulus, a difference of 1.3%. H-300x150x5.5x8 is the lighter of the two at 32 kg/m against 32.8 kg/m, a saving of 2.5% on steel weight. UB 305×102×33 is deeper, 312.7 mm against 298 mm, which matters where the floor zone is tight.
| Property | UB 305×102×33 UB | H-300x150x5.5x8 MB & MC | Difference |
|---|---|---|---|
| Mass | 32.8 kg/m | 32 kg/m | +2.5% |
| Depth | 312.7 mm | 298 mm | +4.9% |
| Width | 102.4 mm | 149 mm | -31.3% |
| Web thickness | 6.6 mm | 5.5 mm | +20.0% |
| Flange thickness | 10.8 mm | 8 mm | +35.0% |
| Area | 41.8 cm² | 40.8 cm² | +2.5% |
| Second moment, major (Iy) | 6,500 cm⁴ | 6,318 cm⁴ | +2.9% |
| Plastic modulus, major (Wpl,y) | 481 cm³ | 475 cm³ | +1.3% |
| Elastic modulus, major (Wel,y) | 416 cm³ | 424 cm³ | -1.9% |
| Second moment, minor (Iz) | 194 cm⁴ | 442 cm⁴ | -56.1% |
| Plastic modulus, minor (Wpl,z) | 60 cm³ | 91.8 cm³ | -34.6% |
| Radius of gyration, minor (iz) | 2.15 cm | 3.29 cm | -34.7% |
| Torsion constant (It) | 12.2 cm⁴ | 8.65 cm⁴ | +41.0% |
UB 305×102×33 to BS EN 10365:2017; H-300x150x5.5x8 to Continental Steel. Difference is expressed relative to H-300x150x5.5x8.
How to choose between them
On stiffness, UB 305×102×33 has the larger second moment of area: 6,500 cm⁴ against 6,318 cm⁴, a difference of 2.9%. This matters more often than the strength comparison, because on normal floor spans deflection rather than bending capacity sets the beam size - so if the two are close on modulus but apart on inertia, the stiffer one is usually the better answer even when it is not the stronger one.
Measured as bending capacity per kilogram, the two are equally efficient (14.7 against 14.8 cm³ per kg/m), so on a pure steel-tonnage basis there is nothing to choose between them. The decision comes down to availability, connection detailing and what the rest of the frame uses.
The two diverge much more about the minor axis: H-300x150x5.5x8 has 91.8 cm³ of minor-axis plastic modulus against 60 cm³, 34.6% more. If the member sees any minor-axis bending, or if it is unrestrained over a long length and lateral-torsional buckling governs, that gap decides the comparison even though the major-axis numbers looked equivalent.
These come from different catalogues, so treat the swap as a design change rather than a like-for-like substitution. UB sections are specified to BS EN 10365:2017 and MB & MC to Continental Steel; the steel grades, tolerances and available lengths differ, connection details and bolt gauges are set out differently, and the two are rarely both stocked in the same market. Check availability before changing a section on a drawing to save 2.5% weight.
In short, UB 305×102×33 is the stronger section and H-300x150x5.5x8 is the lighter one; where those are the same section, it is the clear choice, and where they are not, decide on whether your design is governed by capacity or by tonnage.